[PATCH v3] rcu: ftrace: avoid tracing a few functions executed in stop machine

Patrick Wang
2022-04-26T10:47:01Z

Because of the change of stop machine implementation, there are functions
being called while waiting currently:

----------------------------------------------------------------
Former stop machine wait loop:
do {
cpu_relax(); => macro
...
} while (curstate != STOPMACHINE_EXIT);
-----------------------------------------------------------------
Current stop machine wait loop:
do {
stop_machine_yield(cpumask); => function (notraced)
...
touch_nmi_watchdog(); => function (notraced, inside calls also notraced)
...
rcu_momentary_dyntick_idle(); => function (notraced, inside calls traced)
} while (curstate != MULTI_STOP_EXIT);
------------------------------------------------------------------

These functions (including the calls inside) should be marked notrace to avoid
their codes being updated when they are being called. The calls inside
rcu_momentary_dyntick_idle() still remain traced, and will cause crash:

rcu: INFO: rcu_preempt detected stalls on CPUs/tasks:
rcu: 1-...!: (0 ticks this GP) idle=14f/1/0x4000000000000000 softirq=3397/3397 fqs=0
rcu: 3-...!: (0 ticks this GP) idle=ee9/1/0x4000000000000000 softirq=5168/5168 fqs=0
(detected by 0, t=8137 jiffies, g=5889, q=2 ncpus=4)
Task dump for CPU 1:
task:migration/1 state:R running task stack: 0 pid: 19 ppid: 2 flags:0x00000000
Stopper: multi_cpu_stop+0x0/0x18c <- stop_machine_cpuslocked+0x128/0x174
Call Trace:
Task dump for CPU 3:
task:migration/3 state:R running task stack: 0 pid: 29 ppid: 2 flags:0x00000000
Stopper: multi_cpu_stop+0x0/0x18c <- stop_machine_cpuslocked+0x128/0x174
Call Trace:
rcu: rcu_preempt kthread timer wakeup didn't happen for 8136 jiffies! g5889 f0x0 RCU_GP_WAIT_FQS(5) ->state=0x402
rcu: Possible timer handling issue on cpu=2 timer-softirq=594
rcu: rcu_preempt kthread starved for 8137 jiffies! g5889 f0x0 RCU_GP_WAIT_FQS(5) ->state=0x402 ->cpu=2
rcu: Unless rcu_preempt kthread gets sufficient CPU time, OOM is now expected behavior.
rcu: RCU grace-period kthread stack dump:
task:rcu_preempt state:I stack: 0 pid: 14 ppid: 2 flags:0x00000000
Call Trace:
schedule+0x56/0xc2
schedule_timeout+0x82/0x184
rcu_gp_fqs_loop+0x19a/0x318
rcu_gp_kthread+0x11a/0x140
kthread+0xee/0x118
ret_from_exception+0x0/0x14
rcu: Stack dump where RCU GP kthread last ran:
Task dump for CPU 2:
task:migration/2 state:R running task stack: 0 pid: 24 ppid: 2 flags:0x00000000
Stopper: multi_cpu_stop+0x0/0x18c <- stop_machine_cpuslocked+0x128/0x174
Call Trace:

Mark the calls inside rcu_momentary_dyntick_idle():
rcu_preempt_deferred_qs()
rcu_preempt_need_deferred_qs()
rcu_preempt_deferred_qs_irqrestore()
as notrace to prevent this.

Signed-off-by: Patrick Wang <patrick.wang.shcn@gmail.com>
---
v1->v2:
- Modify log message.

v2->v3:
- Move "notrace" to behind "static" to keep the consistency with the rest of the code.

kernel/rcu/tree_plugin.h | 6 +++---
1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index c8ba0fe17267..440d9e02a26e 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -460,7 +460,7 @@ static bool rcu_preempt_has_tasks(struct rcu_node *rnp)
* be quite short, for example, in the case of the call from
* rcu_read_unlock_special().
*/
-static void
+static notrace void
rcu_preempt_deferred_qs_irqrestore(struct task_struct *t, unsigned long flags)
{
bool empty_exp;
@@ -581,7 +581,7 @@ rcu_preempt_deferred_qs_irqrestore(struct task_struct *t, unsigned long flags)
* is disabled. This function cannot be expected to understand these
* nuances, so the caller must handle them.
*/
-static bool rcu_preempt_need_deferred_qs(struct task_struct *t)
+static notrace bool rcu_preempt_need_deferred_qs(struct task_struct *t)
{
return (__this_cpu_read(rcu_data.cpu_no_qs.b.exp) ||
READ_ONCE(t->rcu_read_unlock_special.s)) &&
@@ -595,7 +595,7 @@ static bool rcu_preempt_need_deferred_qs(struct task_struct *t)
* evaluate safety in terms of interrupt, softirq, and preemption
* disabling.
*/
-static void rcu_preempt_deferred_qs(struct task_struct *t)
+static notrace void rcu_preempt_deferred_qs(struct task_struct *t)
{
unsigned long flags;

--
2.25.1